I will make the not so bold prediction that in terms of life time sales, Gran Turismo Sport will outsell every current Xbox One Forza game, including the Forza Horizon series and including Forza Motorsport 7, combined.

Gran Turismo Sport: 13,000,000

Forza 5: 2.31 Million

Forza 6: 2.00 Million

Forza Horizon 2: 1.41 Million

Forza Horizon 3: 2.52 Million

Forza 7: Unknown (probably 2 million lifetime)

Forza Xbox One games combined = 8,240,000, but we'll add on Forza 7 and get 10,240,000

I planned ahead in anticipation of a post like this, which is why i said "Gran Turismo Sport will outsell every current Xbox One Forza game"

Edit: I forgot to actually answer the question. Unless Gran Turismo Sport is the highest selling GT game ever, which it will have to sell over 15 million to do, or the next 4 Forza games are flops, I can't see any possible way Gran Turismo Sport will outsell all of them combined

I planned ahead in anticipation of a post like this, which is why i said "Gran Turismo Sport will outsell every current Xbox One Forza game"

Edit: I forgot to actually answer the question. Unless Gran Turismo Sport is the highest selling GT game ever, which it will have to sell over 15 million to do, or the next 4 Forza games are flops, I can't see any possible way Gran Turismo Sport will outsell all of them combined

You are essentially predicting this to be the highest selling GT game ever though. You put it at 13m without digital.

Legend11 correctly predicted that GTA IV (360+PS3) would outsell SSBB. I was wrong.